Reference for Rosh Hashanah 25:5

בארבעים סאה הוא טובל בארבעים סאה חסר קורטוב אינו יכול לטבול בהן כביצה מטמא טומאת אוכלין כביצה חסר שומשום אינו מטמא טומאת אוכלין

Said R'Jeremiah to R'Zera: And were the Rabbis certain that there is this distinction between a third and less than a third?<span class="x" onmousemove="('comment',' Viz., that what is grown to a third belongs to one year, and what is less grown to another year. This seems to R. Jeremiah rather arbitrary.');">⁵</span> He replied to him: Am I not always telling you not to let yourself go beyond the established rule?

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